While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, 30, 322–334, (1969); and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often greater than 100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g., less than 1.0 μm) between the anode and the cathode. Herein, the term “organic EL element” encompasses the layers between the anode and cathode. Reducing the thickness lowered the resistance of the organic layer and has enabled devices that operate at much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, and therefore, it is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons, and is referred to as the electron-transporting layer. Recombination of the injected holes and electrons within the organic EL element results in efficient electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by Tang et al (J. Applied Physics, 65, Pages 3610–3616, (1989)). The light-emitting layer commonly consists of a host material doped with a guest material, also known as a dopant. Still further, there has been proposed in U.S. Pat. No. 4,769,292 a four-layer EL element comprising a hole-injecting layer (HIL), a hole-transporting layer (HTL), a light-emitting layer (LEL) and an electron transport/injection layer (ETL). These structures have resulted in improved device efficiency.
Since these early inventions, further improvements in device materials have resulted in improved performance in attributes such as color, stability, luminance efficiency and manufacturability, e.g., as disclosed in U.S. Pat. Nos. 5,061,569, 5,409,783, 5,554,450, 5,593,788, 5,683,823, 5,928,802, 6,020,078, and 6,208,077, amongst others.
Notwithstanding these developments, there are continuing needs for organic EL device components, such as new light-emitting materials, and methods for incorporating them into an EL device.
Tris(2-methyl-8-quinolinolato)aluminum(III), Alq3, has been used extensively in electroluminescent devices because of its ability to transport electrons. It has been used as a yellow-green emissive material as well as a host for emissive dopants. Tris-chelated octahedral complexes, such as Alq3, can exist in the meridional (A) or facial (B) isomeric forms. FIG. 1 show a simple picture of these two forms. The meridional-isomer of Alq3 commonly crystallizes in either of two polymorphs often called the β or ε-phases. These forms will be referred to as β- and ε-Alq3.
Until very recently, only the meridional form of Alq3 had been fully characterized, although, M. Yasushi, S. Kenji, U. Taeko (JP 2902745) and S. Kenji, K. Yasushi, U. Taeko, M. Yasushi, (JP 2823352) had reported a green emitting Alq3 structure that they proposed as a facial-isomer.
Only recently the facial-isomer of δ-Alq3 has been identified and completely characterized and this form of Alq3 emits blue light. Two phases (δ and γ) of Alq3 that can be formed at higher temperatures have been characterized using X-ray diffraction technique as the primary characterization method. [“Refinement of the Crystal Structure of the δ-Modification of tris(8-hydroxyquinoline) Aluminum(III), —Al(C9H6NO)3, the blue luminescent Alq3: Manju Rajeswaran, Thomas N. Blanton and Kevin P. Klubek, Z. Kristallogr. NCS 218, 439–440, 2003.] Recently M. Colle, J. Gmeiner, W. Milius, H. Hillebrecht, and W. Brutting, Adv. Funct. Mater., 13, 108 (2003) have also reported the δ form of Alq3. Colle and co-workers report that for annealing temperatures up to 365° C., samples of α-Alq3 are a yellowish-green powder with a photoluminescence maximum of 506 nm. After an exothermic transition at about 380° C. they report the formation of the blue-light emitting Alq3.
The facial-isomer of δ-Alq3 is very interesting due to its blue emission since it is known that other colors, such as green and red emission can be obtained from blue emitting materials by means of energy transfer to the appropriate dopant. However it has been very difficult to produce Alq3 in the facial form in large quantities and in a pure form. Purity is important since, in certain cases, when the facial-isomer of Alq3 is mixed with the meridional-isomer in a continuous film, energy transfer can occur from the blue emitting form to the green emitting phase resulting in a less desirable green emission by the mixture. It would also be desirable to have a process for producing only one polymorph of the facial-isomer of Alq3 since it is known that polymorphs can have different physical properties and this can lead to problems in manufacturing a product.
It is not possible to deposit films of the blue-emitting facial-isomer of Alq3 via common vapor deposition methods because heating the facial-isomer, such as the δ-Alq3, converts this material back into the α-Alq3 phase. Colle and co-workers do report evaporating thin films of α-phase Alq3 and then converting this material to the δ-phase by annealing the film at 390° C. between two glass plates. This is not a practical procedure for preparing an EL device since glass does not form a conforming layer on the Alq3 and large variations in layer thickness of the deposited δ-phase layer would be anticipated.
Recently M. Muccini and co-workers (WO 2003/106422) also described a process for the preparation of the facial-isomer of Alq3 as a mixture of two polymorphs, the γ and δ-phases of Alq3, by heating the crystalline α-phase to very high temperatures. They report that, starting from the α-phase of Alq3, the solid state transformation of meridional-isomer into facial-isomer occurs only near 390° C. They describe heating commercial α-Alq3 to 395° C. to form a mixture of γ and δ-Alq3 wherein the ratio of γ-phase to the δ-phase is about 10/1 and that this ratio is not significantly changed by differences in heating and cooling rates or by heating at a temperature of 410° C. instead of 395° C.
Solution coatings of a mixture of the γ- and δ-phases of Alq3 are also reported M. Muccini et al. to form films, deposited on quartz substrates, that emit blue light. Solutions of the facial-isomer of Alq3 are kept at low temperature to prevent-isomerization to the meridional-isomer. However, the solution processing described is not readily applied to the solid-state deposition used in the formation of many EL devices.
It is a problem to be solved to provide an economical process for forming thin films of the facial form of an aluminum trisquinoline complex, such as δ-Alq3, that would not require extremely high temperatures and that would provide films of high purity and integrity that emit blue light. In addition, it is desirable to find a means to provide composites comprising such films so that the composites dimensions are thermally stable and of uniform composition. Such composites would be suitable for use in an EL device, especially an OLED device which is formed by vapor-phase deposition.